Switch L2 images to use Debian Backports instead of Rustup

[alexhulbert]

The new Debian-backports rust versions:

• Are scoped to particular images, meaning updating rust on Bob L2 will not change the rust version on the L2 op-rbuilder image, etc
• Are pinned to the Debian repo timestamp. The image measurements will stay the same even if a new Rust version comes out

These changes:

• speed up build times
• remove the rust toolchain from our reproducibility threat model (since the debian backports version can be independently verified via the deb rebuilder scripts)
• remove the unnecessary network calls to download the entire rust toolchain on each build (regardless of cache)

To update rustc/cargo to the latest version for a particular image, you only need to update the Debian repo timestamp for the image's root config file (e.g. the timestamp in l2/_op_rbuilder/mkosi.conf). Rust maintains best-in-class backwards compatibility, which means granularity of specific rust versions are unnecessary. Updating to the latest version based on timestamp should always be sufficient.